composition of matter in chemistry

ATOM

            An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are very small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale).

               Atoms are small enough that attempting to predict their behavior using classical physics - as if they were billiard balls, for example - gives noticeably incorrect predictions due to quantum effects. Through the development of physics, atomic models have incorporated quantum principles to better explain and predict the behavior.

Most of the Universe consists of matter and energy. Energy is the capacity to do work. Matter has mass and occupies space. All matter is composed of basic elements that cannot be broken down to substances with different chemical or physical properties. Elements are substances consisting of one type of atom, for example Carbon atoms make up diamond, and also graphite. Pure (24K) gold is composed of only one type of atom, gold atoms. Atoms are the smallest particle into which an element can be divided. The ancient Greek philosophers developed the concept of the atom, although they considered it the fundamental particle that could not be broken down. Since the work of Enrico Fermi and his colleagues, we now know that the atom is divisible, often releasing tremendous energies as in nuclear explosions or (in a controlled fashion in) thermonuclear power plants.

Subatomic particles were discovered during the 1800s. For our purposes we will concentrate only on three of them, summarized in Table 1. The proton is located in the center (or nucleus) of an atom, each atom has at least one proton. Protons have a charge of +1, and a mass of approximately 1 atomic mass unit (amu). Elements differ from each other in the number of protons they have, e.g. Hydrogen has 1 proton; Helium has 2.

                   The neutron also is located in the atomic nucleus (except in Hydrogen). The neutron has no charge, and a mass of slightly over 1 amu. Some scientists propose the neutron is made up of a proton and electron-like particle.

           The electron is a very small particle located outside the nucleus. Because they move at speeds near the speed of light the precise location of electrons is hard to pin down. Electrons occupy orbitals, or areas where they have a high statistical probability of occurring. The charge on an electron is -1. Its mass is negligible (approximately 1800 electrons are needed to equal the mass of one proton).

Table 1. Subatomic particles of use in biology.

Name	Charge	Location	Mass
Proton	+1	atomic nucleus	1.6726 X 10-27 kg
Neutron	0	atomic nucleus	1.6750 X 10-27 kg
Electron	-1	electron orbital	9.1095 X 10-31 kg

The atomic number is the number of protons an atom has. It is characteristic and unique for each element. The atomic mass (also referred to as the atomic weight) is the number of protons and neutrons in an atom. Atoms of an element that have differing numbers of neutrons (but a constant atomic number) are termed isotopes. Isotopes, shown in Figure 1 and Figure 2, can be used to determine the diet of ancient peoples by determining proportions of isotopes in mummified or fossilized human tissues. Biochemical pathways can be deciphered by using isotopic tracers. The age of fossils and artifacts can be determined by using radioactive isotopes, either directly on the fossil (if it is young enough) or on the rocks that surround the fossil (for older fossils like dinosaurs). Isotopes are also the source of radiation used in medical diagnostic and treatment procedures.

Figure 1. Note that each of these isotopes of hydrogen has only one proton. Isotopes differ from each other in the number of neutrons, not in the number of protons. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

            Some isotopes are radioisotopes, which spontaneously decay, releasing radioactivity. Other isotopes are stable. Examples of radioisotopes are Carbon-14 (symbol ¹⁴C), and deuterium (also known as Hydrogen-2; ²H). Stable isotopes are ¹²C and ¹H.

Figure 2. Carbon has three isotopes, of which carbon-12 and carbon-14 are the most well known. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The Periodic Table of the Elements, a version of which is shown in Figure 3, provides a great deal of information about various elements. An on-line Periodic Table is available by clicking here,

Figure 3. The Periodic Table of the Elements. Each Roman numeraled column on the label (at least the ones ending in A) tells us how many electrons are in the outer shell of the atom. Each numbered row on the table tells us how many electron shells an atom has. Thus, Hydrogen, in column IA, row 1 has one electron in one shell. Phosphorous in column VA, row 3 has 5 electrons in its outer shell, and has three shells in total. Image from James K. Hardy's chemistry site at the University of Akron.

ELEMENT Definition of Element What is an Element?                 An element is a substance consisting of atoms which all have the same number of protons - i.e. the same atomic number.           Elements are chemically the simplest substances and hence cannot be broken down using chemical methods. Elements can only be changed into other elements using nuclear methods.          Although an element's atoms must all have the same number of protons, they can have different numbers of neutrons and hence different masses. When atoms of the same element have different numbers of neutrons, they are called isotopes.                         An element or chemical element is the simplest form of matter in that it cannot be further broken down using any chemical means. Yes, elements are made up of smaller particles, but you can't take an atom of an element and perform any chemical reaction that will break it apart or join its subunits to make a bigger atom of that element. Atoms of elements may be broken down or fused together using nuclear reactions.                So far, 118 chemical elements have been found. Of these, 94 are known to occur in nature, while the others are man-made or synthetic elements. 80 elements have stable isotopes, while 38 are purely radioactive. The most abundant element in the universe is hydrogen. In the Earth (as a whole), it's iron. In the Earth's crust and human body, the most abundant element by mass is oxygen. What Makes Elements Different From Each Other?                  So, you may be asking yourself what makes one material a different element from another? How can you tell if two chemicals are the same element?                  Sometimes examples of a pure element look very different from each other. For example, diamond and graphite (pencil lead) are both examples of the element carbon. You wouldn't know it based on appearance or properties. However, atoms of diamond and graphite each share the same number of protons. The number of protons, particles in an atom's nucleus, determines the element. Elements on the periodic table are arranged in order of increasing numbers of protons. The number of protons is also known as an element's atomic number, which is indicated by the number Z. The reason different forms of an element (called allotropes) can have different properties even though they have the same number of protons is because the atoms are arranged or stacked differently. Think of it in terms of a set of blocks. If you stack the same blocks in different ways, you get different objects. Examples of Elements Pure elements can be found as atoms, molecules, ions, and isotopes. So, examples of elements include a hydrogen atom (H), hydrogen gas (H2), the hydrogen ion H+, and isotopes of hydrogen (protium, deuterium, and tritium). The element with one proton is hydrogen. Helium contains two protons and is the second element. Lithium has three protons and is the third element, and so on. Hydrogen has the smallest atomic number (1), while the largest known atomic number is that of the recently discovered element oganesson (118).            Pure elements contain atoms that all have the same number of protons. If the number of protons of the atoms in a sample is mixed, you have a mixture or a compound. Examples of pure substances that are not elements include water (H2O), carbon dioxide (CO2) and salt (NaCl). See how the chemical composition of these materials includes more than one type of atom? If the atoms had been the same type, the substance would have been an element even though it contained multiple atoms. Oxygen gas, (O2) and nitrogen gas (N2) are examples of elements.                                    MOLECULE The terms molecule, compound, and atom can be confusing! Here's an explanation of what a molecule is (and is not) with some examples of common molecules. Molecules form when two or more atoms form chemical bonds with each other. It doesn't matter if the atoms are the same or are different from each other. Examples of Molecules Molecules may be simple or complex. Here are examples of common molecules: H2O (water) N2 (nitrogen) O3 (ozone) CaO (calcium oxide) C6H12O6 (glucose, a type of sugar) Molecules Versus Compounds Molecules made up of two or more elements are called compounds. Water, calcium oxide, and glucose are molecules that compound. All compounds are molecules; not all molecules are compounds. What Is Not a Molecule? Single atoms of elements are not molecules. A single oxygen, O, is not a molecule. When oxygen bonds to itself (e.g., O2, O3) or to another element (e.g., carbon dioxide or CO2), molecules are formed. COMPOUND                     In chemistry, a compound is a substance that results from a combination of two or more different chemical element s, in such a way that the atom s of the different elements are held together by chemical bonds that are difficult to break. These bonds form as a result of the sharing or exchange of electron s among the atoms. The smallest unbreakable unit of a compound is called a molecule .                 A compound differs from a mixture, in which bonding among the atoms of the constituent substances does not occur. In some situations, different elements react with each other when they are mixed, forming bonds among the atoms and thereby producing molecules of a compound. In other scenarios, different elements can be mixed and no reaction occurs, so the elements retain their individual identities. Sometimes, when elements are mixed, the reaction occurs slowly (as when iron is exposed to oxygen); in other cases it takes place rapidly (as when lithium is exposed to oxygen). Sometimes, when an element is exposed to a compound, a reaction occurs in which new compounds are formed (as when pure elemental sodium is immersed in liquid water).             Often, a compound looks and behaves nothing like any of the elements that comprise it. Consider, for example, hydrogen (H) and oxygen (O). Both of these elements are gases at room temperature and normal atmospheric pressure. But when they combine into the familiar compound known as water, each molecule of which contains two hydrogen atoms and one oxygen atom (H 2 O), the resulting substance is a liquid at room temperature and normal atmospheric pressure. The atoms of a few elements do not readily bond with other elements to form compounds. These are called noble or inert gases: helium, neon, argon, krypton, xenon, and radon. Certain elements readily combine with other elements to form compounds. Examples are oxygen, chlorine, and fluorine. 2) As an adjective, compound is the quality of being composed of multiple parts. Examples of this usage include compound eye (found in various insects), compound microscope (a high-power magnifying instrument consisting of more than one lens), compound sentence (a sentence that is a conjunction or disjunction of smaller sentences), and compound document , an organized collection of user interfaces that form a single integrated perceptual environment. REFERENSI https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookCHEM1.html https://en.wikipedia.org/wiki/Atom https://www.thoughtco.com/what-are-elements-608810  https://www.thoughtco.com/what-is-a-molecule-definition-examples-608506